Mismatch repair is an error avoidance pathway devoted to enhancing the fidelity of DNA replication and maintaining genetic stability. Mutations in mismatch repair genes predispose individuals to cancer. The 8-oxoG lesion is a major stable product of oxidative damage, has the most deleterious effects, and is a substrate for the E. coli Mut Y protein. The functional groups of the Mut Y protein for substrate specificity and catalysis will be defined. This will be achieved by site-directed mutagenesis of the mut Y gene and construction of chimeric proteins of Mut Y and GTMR. The functional roles of the C-terminal domain of Mut Y will also be elucidated. The structure of the 15 kDa terminal fragment will be determined by NMR. Possible interactions between Mut Y and Mut M will be investigated. Purified SpMYH protein of fission yeast will be assayed on DNA containing mismatches. The effects of overexpression of SpMYH in E. coli and yeast will be investigated. Yeast mutants defective in myh will be constructed and their mutation rates will be measured.